Silicones are widely used in the electrical and electronics industries as a result of their unique properties. Silicones exhibit low alpha particle emissions, very good moisture resistance, excellent electrical insulation, excellent thermal stability, and very high ionic purity. In particular, silicone encapsulants can improve the reliability of an electronic device by providing an effective barrier against environmental moisture, UV radiation, ozone, and weathering.
Moreover, recent advances in semiconductor packaging, namely, the development of chip scale or chip size packages, have created a critical demand for high performance, vacuum dispensable silicone encapsulants. In addition to the aforementioned properties of electronic grade silicone materials, such encapsulants must be compatible with the new vacuum dispensing systems and possess the rheological properties required for flow around and/or under the silicon chip or die.
Addition-curable silicone compositions comprising an alkenyl-containing polydiorganosiloxane, an organopolysiloxane resin, an organohydrogenpolysiloxane crosslinking agent, and a hydrosilylation catalyst are well known in the art. Illustrative of such compositions are U.S. Pat. No. 4,427,801 to Sweet; U.S. Pat. No. 4,500,659 to Kroupa et al.; U.S. Pat. No. 4,882,398 to Mbah; U.S. Pat. No. 5,519,082 to Yoshino; and U.S. Pat. No. 4,082,726 to Mine et al.
However, conventional silicone compositions, including the preceding, are unsuitable for vacuum dispensing processes used in the fabrication of chip scale packages, due at least in part to excessive outgassing. Conventional silicone compositions evolve copious amounts of air during vacuum dispensing. Also, low boiling components in the compositions, either initially present or later formed during storage, also contribute to gas evolution. The rapidly escaping gas bubbles cause foaming and splattering of the encapsulant, resulting in contamination of the exposed surface of the semiconductor device. An additional cleaning step is required to remove encapsulant from the contaminated die surface. Moreover, extensive gas evolution produces voids in the encapsulant layer, resulting in incomplete underfill of the device. Contamination and residual voids become increasingly conspicuous as the complexity of the device increases and its dimensions decrease. In the fabrication of chip scale or chip size semiconductor packages, these encapsulation problems result in increased costs and reduced component reliability.